I'd take the 600. I've always loved the sound of all of the SCI synths. The 106 is a great synth but you see them every where. Every time someone sees my studio they ask "what is that" when they see my SCI Six-Trk. You can get some of the craziest bass sounds out of the Curtis chips.
THM is absolutely right. Really depends on how you play it. I have both. The P600 is 2 osc while the juno has one with a sub. The juno to me is smoother while the P600 can growl. Both are so cheap now. My P600 is damaged but a replacement is on the horizon. I have two Junos. I love my Juno. I've had it for 20 years when the first came out. Just think every salesman was pushing the DX7. I think knowing what I know now. I would go with P600. Actually, that's really tough. As soon as I typed that I felt regret. You can't lose with either. If midi is a big thing Juno. 2osc analog bliss-P600.
I always wanted to pick up a Juno at some point, but actually came accross a P600 aquiring old studio gear. I don't really have the need for the Juno anymore (voyager for bass, mmmm). Seems to be worth a bit more as well.